1. Field of the Invention
The present invention relates to an image display apparatus for displaying an image by applying a laser beam to a projection surface.
2. Description of the Background Art
<Full-Color Laser Projector>
In order to reproduce full colors by a so-called laser projector that displays an image by applying a laser beam to a projection surface such as screen or wall, it is necessary to prepare laser beams with the three primary colors of red, green and blue.
For example, Japanese Patent Laying-Open No. 2003-021800 discloses a projection-type display apparatus scanning light beams from three independent light sources toward a projection surface and splitting the light beams between pixels while scanning the beams so that the light beams are radiated as pulsed beams.
Japanese Patent Laying-Open Nos. 2004-219480 and 2006-154032 also disclose respective display apparatuses using laser beams of three colors. In the display apparatuses disclosed in these documents each, three independent light sources are used for generating laser beams of three colors.
Further, Japanese Patent Laying-Open No. 09-152640 discloses a laser apparatus converting wavelengths of a laser beam from a single laser by three wavelength-converters to generate laser beams of three colors. The laser apparatus cannot emit the laser beams of three colors at exactly the same time. Therefore, the laser apparatus includes a wavelength-converter holding plate where the three wavelength-converters are arranged on the same circumference, and a rotating drive unit rotating the wavelength-converter holding plate, so that the human eyes recognize the light of three colors in synthesized state.
Respective apparatuses disclosed in Japanese Patent Laying-Open Nos. 2003-021800, 2004-219480 and 2006-154032 each require many components and many adjustments for obtaining three-color laser beams. This will be described with reference to FIG. 1. FIG. 1 is a diagram for illustrating an optical system for obtaining three-color laser beams in the conventional technique (hereinafter referred to as conventional optical system).
As shown in FIG. 1, the conventional optical system includes a blue laser 102, a red laser 104, a green laser 106, a first PBS (Polarized Beam Splitter) 109, a second PBS 110, a collimator lens 112, a two-axis galvanometer mirror 114, a group of lenses 116, and a screen 118. Further, green laser 106 includes an infrared laser 107 and a second-harmonic generation (SHG) element 108.
Here, the following description also applies to an apparatus using, instead of the above-described elements, other elements having similar functions. For example, the following description applies as well to an apparatus using a green laser configured differently from the above-described one.
Blue laser 102, red laser 104 and green laser 106 emit laser beams of blue, red and green respectively. First PBS 109 causes respective optical paths of the red laser beam and the green laser beam to coincide with each other. Second PBS 110 causes respective optical paths of the light emitted from first PBS 109 and the blue laser beam to coincide with each other. Collimator lens 112 converges the light emitted from second PBS 110 and makes the light parallel. Two-axis galvanometer mirror 114 scans the light passed through collimator lens 112. The group of lenses 116 focuses the light reflected from galvanometer mirror 114 on screen 118. Screen 118 is a projection surface for the laser beam.
As seen from the above, the conventional optical system requires many components such as the three lasers and the two PBSs used for causing respective optical paths of the beams from respective lasers to coincide with each other. Accordingly, the projector is large in size. Further, because of the use of the two PBSs, the transmission rate of the laser beam lowers and the light quantity decreases. Furthermore, it is necessary to adjust many elements for causing laser beams to coincide with each other.
In the case where the laser apparatus disclosed in Japanese Patent Laying-Open No. 09-152640 is used instead of the three lasers, the PBS is unnecessary. This method, however, requires the wavelength-converter holding plate and the rotating drive unit and thus also requires many components.
<Detection of Scan Abnormality>
In an apparatus displaying an image by applying a laser beam to a projection surface such as screen, namely so-called laser projector, if an abnormality occurs in scanning of the laser beam, the laser beam of a high energy density is continuously applied to a certain point on the screen, which is dangerous. Depending on the case, the danger could be posed to any person around the apparatus.
Some techniques for avoiding such a danger have been proposed. For example, a laser projector disclosed in Japanese Patent Laying-Open No. 2004-341210 applies infrared light to a galvanometer mirror used as a scan element, and detects the reflected light thereof by a plurality of light-receiving elements provided along the outer periphery of the screen. Then, the laser projector analyzes the state of scanning from the detected state, and stops applying a laser beam when an abnormality in scanning is found.
Further, an image display apparatus disclosed in Japanese Patent Laying-Open No. 2007-003687 is provided with a light-receiving unit in an overscan area of a surface to be scanned. When a light beam is scanned on the surface to be scanned, the image display apparatus determines whether or not a signal from the light-receiving unit is output at a certain time interval, and controls emission of the light beam according to the result of the determination.
Further, a projection display apparatus disclosed in Japanese Patent Laying-Open No. 2000-194302 monitors the scan timing of an actually emitted laser beam by a main scan beam sensor provided near a main scan polygon mirror. When the timing is abnormal, the projection display apparatus turns off the laser.
The apparatuses disclosed in above-referenced Japanese Patent Laying-Open Nos. 2004-341210 and 2007-003687 each find an abnormality in scanning, from a signal provided from the light-receiving means provided on the outer periphery of the screen. The apparatus disclosed in Japanese Patent Laying-Open No. 2000-194302 detects an abnormality in scanning by detecting the reflected light from the main scan polygon mirror. The apparatuses, however, cannot monitor the state of scanning all the time. Therefore, a long time could pass from the time when an abnormality actually occurs to the time when the abnormality is detected. Further, the precision in detection of an abnormality of these apparatuses is low.
<Collimation of Laser Beam>
For an apparatus displaying an image by applying a laser beam to a projection surface such as screen, namely so-called laser projector, it is necessary that a laser beam emitted from a laser is parallel light with a very small diameter.
A collimator lens is an element often used for obtaining parallel light. For example, Japanese Patent Laying-Open No. 2003-021800 discloses a projection-type display apparatus including a collimator lens.
Further, a laser beam correction mechanism for obtaining parallel light is disclosed in Japanese Patent Laying-Open No. 05-291659. The laser beam correction mechanism includes two beam imaging units and an image processing unit. The two beam imaging units are respectively provided at two positions apart from each other on a track of a laser, and the imaging units output respective beam projection images at their positions. The image processing unit processes respective image signals obtained by the two beam imaging units, and outputs a signal for controlling the positions of mirrors and lenses.
The above-described method using the collimator lens, however, cannot always obtain parallel light due to a change in condition such as temperature.
Further, the laser beam correction mechanism disclosed in Japanese Patent Laying-Open No. 05-291659 is large in scale due to the requirement of two beam imaging units for example.